Over the past two decades, significant effort has been devoted to the regenerable hot-gas desulfurization from coal-derived fuel gas streams at high temperatures. See Flytzani-Stephanopoulos, M and Li, Z., “Kinetics of Sulfidation Reactions Between H2S and Bulk Oxide Sorbents,” Invited review paper, NATO-Advanced Study Institute on “Desulfurization of Hot Coal Gas”, NATO ASI Series G, v. 42 (ed. A. T. Atimtay, D. P. Harrison), pp. 179-212. Bulk metal oxides have been studied more extensively than supported oxides as potential hot gas desulfurization sorbents due to lower manufacturing costs and higher sulfur capacity (grams of sulfur per grams of sorbent).
The general desulfurization reaction of a metal oxide sorbent in a reducing environment can be written as follows:MyO+H2S+H2 (or CO)→MyS+H2O (or CO2)
Zinc oxide is the sorbent of choice in non-regenerable schemes and for operations at low temperatures (<400° C.). At higher temperatures (˜550° C.), zinc-oxide gets reduced to volatile Zn in a reformate gas stream. See Flytzani-Stephanopoulos, M., “Alternative Sorbent Development,” Keynote lecture at DOE/METC Workshop on “Status and Direction of Research and Development for High Temperature Sulfur Removal Sorbents,” Morgantown, W. Va., Jan. 28, 1993; Lew, S., Sarofim, A. F., and Flytzani-Stephanopoulos, M., Chem. Eng. Sci. 47 (6), 1421-1431 (1992); and Lew, S., Sarofim, A. F., and Flytzani-Stephanopoulos, M., Ind. Eng. Chem. Res. 31, 1890-1899 (1992). Mixed oxide compounds of zinc (e.g., zinc ferrite and zinc titanate) with less zinc activity are more suitable for higher temperatures but present problems in implementation due to “stratified” sulfidation/reduction in fixed-bed designs. See Gasper-Galvin, L. “Review of METC Test Program,” in DOE/METC Workshop on “Status and Direction of Research and Development for High Temperature Sulfur Removal Sorbents,” Morgantown, W. Va., Jan. 28, 1993. Zinc migration occurs to the surface of the sorbent particles. The zinc metal either volatilizes or forms dense ZnS resulting in very slow diffusion inwards, i.e., low sulfur capacity and difficulty in regeneration. See Flytzani-Stephanopoulos, M., “Alternative Sorbent Development,” Keynote lecture at DOE/METC Workshop on “Status and Direction of Research and Development for High Temperature Sulfur Removal Sorbents,” Morgantown, W. Va., Jan. 28, 1993. The use of copper-based sorbents, has been also advocated. See Flytzani-Stephanopoulos, M., Yu, T. U., and Lew, S., “Development and Testing of Desulfurization Sorbents,” Topical Report to Texaco, under subcontract, DOE Coop. Agreement No. DE-FC21-87MC23277, Dec. 1988; Patrick, V., Gavalas, G. R., Flytzani-Stephanopoulos, M., and Jothimurugesan, K., Ind. Eng. Chem. Res. 28, 931-340 (1989); and Li, Z., and Flytzani-Stephanopoulos, M., “Cu—Cr—O and Cu—Ce—O Regenerable Sorbents for Hot Gas Desulfurization,” Ind. Eng. Chem. Res. 36, 187-196 (1997). In particular, for a fixed-bed operation at temperatures exceeding 700° C., the combination of copper with chromium oxide and cerium oxide has been recently shown to be most appropriate. See Li, Z., and Flytzani-Stephanopoulos, M., “Cu—Cr—O and Cu—Ce—O Regenerable Sorbents for Hot Gas Desulfurization,” Ind. Eng. Chem. Res. 36, 187-196 (1997). Iron and aluminum oxides can also be combined with copper oxide to stabilize the latter against total reduction to the metal form, but they are less effective stabilizers than chromium oxide. Another candidate is Cu—CeO2, which has several advantages. CeO2 does not stabilize CuO against reduction; it actually enhances the CuO reducibility. See Liu, W., and Flytzani-Stephanopoulos, M., The Chem. Eng. J. 64, 283 (1996); and Liu, W., Wadia, C., and Flytzani-Stephanopoulos, M., Catal. Today 28 (4), 391 (1996). However, CeO2 is an excellent dispersant keeping copper in finely divided form. In turn, copper increases the reducibility of ceria. The reduced form Ce2O3 is a highly efficient sorbent. See Kay, D. A. R., et al., U.S. Pat. No. 4,826,664 (1989). The sulfur capacity of the Cu—CeO2 system is higher than that of Cu with other stabilizers since CeO2 can also participate in desulfurization through the formation of Ce2O2S and Ce2S3 phases.
Future methods of production of electricity utilize processes such as Integrated Gasifier Combined Cycle (IGCC) systems or fuel cells or pressurized fluid bed combustion. In these methods of electric power production, the reaction of coal with oxygen is not carried to completion. As a result, the gases contain amounts of hydrogen (H2) and carbon monoxide (CO) which are generally greater than the amount of carbon dioxide (CO2) and H2O in these gases. The sulfur from the coal used to produce these gases is mainly in the form of hydrogen sulfide (H2S) or sulfur carbonyl (COS). Such gases are hereinafter referred to as “fuel” gases.
Desulfurization to the lowest possible levels of fuel gases is critical because of restrictions on the amount of sulfur released into the atmosphere from the combustion of coal. The restrictions have been imposed by the Federal Clean Air Act, the provisions of which are enforced by the Environmental Protection Agency. Desulfurization to these low levels is also required for efficient, long term operation of IGCC systems and fuel cells.
Desulfurization at the highest possible rate is also important. The rate of desulfurization will control the size of the equipment used in which desulfurization of fuel or flue gases is conducted. Smaller sized reaction vessels will reduce the capital cost for the desulfurization of gases. It is also important that the utilization of the sorbent be as high as possible over many cycles of sulfidation and regeneration to minimize the amount of sorbent required. While adsorption of H2S is known to take place on oxide sorbents, such as cerium oxide, the resulting sulfided sorbent is difficult to regenerate fully. In fixed bed reactors, the process is slow and complicated by the production of a mixture of sulfur-containing gases that cannot be readily processed to recover the sulfur value. The problem of highly efficient and regenerative hot desulfurization of gas streams remains an unsolved problem.
Presently, fuel cells are undergoing rapid development for both stationary and transportation applications. Low-sulfur diesel and fuel oils are being contemplated for use with fuel cells domestically. However, even with low-sulfur fuels, there should be a sorbent unit effectively removing H2S upstream of the fuel cell to protect the anode material from sulfur poisoning. A lot of interest in this field derives from the intended use of fuel processing to produce hydrogen for fuel cells. Any sulfur present in the fuel will be converted to H2S during the auto-thermal or stream reforming step of fuel processing. The sensitivity of most anode materials to sulfur requires deep desulfurization of the anode feed gas stream. For high temperature fuel cells, such as solid oxide fuel cells (SOFC), the desulfurization unit will operate at temperatures exceeding 600° C. A sorbent is needed with high structural stability in cyclic operation at these temperatures.
The application of lanthanide oxides to substrates for desulfurization of fuel gases has been described by Wheelock et al., U.S. Pat. Nos. 3,974,256 and 4,002,720. However, Wheelock et al. fails to successfully solve the problem that, (1) during regeneration of lanthanide sulfides or lanthanide oxy-sulfides other than cerium sulfide or cerium oxysulfide, lanthanide oxysulfate could be formed which would require temperatures in excess of 1500° C. to regenerate back to lanthanide oxide, and (2) in many cases the utilization of the sorbent for desulfurization would be reduced to a small fraction of its original utilization because of the formation of these lanthanide oxy-sulfates and lanthanide sulfates.
Furthermore, Wheelock et al. utilizes alkali or alkaline earth metal components (as oxides). Thus, the prior art, including Wheelock et al., failed to appreciate that the low melting point oxides of the alkalis would react with the lanthanide oxides to create a mixture which may not be capable of reacting with the sulfur in either fuel or flue gases. The cerium oxide sorbents of the present invention avoid this problem by doping the cerium oxide with lanthanide or transition metal oxides such as copper oxide.
The application of cerium oxide coatings to substrates for the desulfurization of fuel gases has been suggested by Kay et al, U.S. Pat. No. 4,885,145. The information in Column 6, lines 3 through 7 of Kay et al. acknowledges that putting cerium oxide on a support would increase its utilization. Kay et al. states that increasing the utilization of the sorbent also increases the rate of desulfurization and the extent of desulfurization. However, Kay et al. does not solve the long regeneration times that do not match the sulfiding times which require multiple units.
Longo, U.S. Pat. Nos. 4,001,375 and 4,251,496, describes the use of cerium oxide for the desulfurization of flue gases. The methods utilized by Longo to apply the cerium oxide to an Al2O3 support are described in detail in these patents. However, Longo does not teach or suggest effective desulfurization sorbents at low temperatures.
Kay et al., U.S. Pat. No. 4,885,145, describes the utilization of solid solutions of cerium oxide and other altervalent oxides of either other lanthanides or oxides of the alkaline earth elements to increase the utilization of the sorbents, which are solid solutions, as well as to increase the extent of desulfurization and the rate of desulfurization of fuel gases. However, Kay et al. does not disclose using copper oxides in the cerium oxide sorbents or effective desulfurization at low temperatures.
Koberstein et al., U.S. Pat. No. 5,024,985, describes a support material for a three-way automotive catalyst containing platinum group metal and having a reduced tendency for H2S emissions. The support material is formed from an annealed spray-dried combination of aluminum oxide and cerium oxide. In the process described in Koberstein et al., SO2 in the exhaust gas exiting the engine reacts under oxidizing conditions (λ=1.02) with the CeO2 portion of the catalyst to form Ce2(SO4)3. When a reducing gas (λ=0.92) is passed over the Ce2(SO4)3, a release of H2S and SO2 occurs with the regeneration of Ce2(SO4)3 back to CeO2, which is again capable of reacting with the SO2 in an oxidizing gas (λ=1.02). The reaction for the release of SO2 and H2S during regeneration of Ce2(SO4)3 has been described in the Longo patents previously cited.
Koberstein et al. does not teach or suggest that the CeO2 portion of the catalyst reacts with H2S in the automobile exhaust gas. In fact, the exhaust gas exiting the automobile engine does not contain H2S. Rather, the data of Koberstein et al. shows in the Examples provided therein that the smaller surface area of the CeO2 portion of the catalyst annealed at 1000° C. limits the amount of SO2 that reacts with the CeO2 to form Ce2(SO4)3, thereby limiting the amount of H2S which may be subsequently emitted as a result of the chemically reducing action of the %=0.92 gas with Ce2(SO4)3.
Koberstein et al. illustrates this principle in Comparative Example 1 and Example 3. In Comparative Example 1, high surface area is maintained by a final annealing step in hydrogen at 550° C. for four hours. In Example 3, Koberstein et al. prepares the catalyst in the same manner as Comparative Example 1 except that the final annealing step is performed at 1000° C. for 24 hours in hydrogen. It is known to those skilled in the art that the surface area of CeO2 is markedly reduced by annealing at temperatures as high as 1,000° C. This is particularly true when the annealing step is performed in an atmosphere of hydrogen, which is necessary to reduce the hexachloroplatinic salt to platinum metal.
Wilson et al., U.S. Published Patent Application No. 2002/0044901, discloses a method of desulfurizing gases in which microdomains or microcrystals of cerium oxide are provided with an aluminum oxide substrate. Wilson et al. reports that the use of microdomains provides a high surface area of cerium oxide, and a stable surface area of cerium oxide, which react in a rapid fashion with sulfur compounds within the fuel gas. However, forming the microdomains and microcrystals on an aluminum oxide substrate requires extra formation steps that may not be practical on a production scale.
Effective regeneration of fully sulfided sorbents is fraught with problems. These problems include 1) long regeneration times not matching the sulfidation times necessitate the use of multiple units, greatly increasing the weight/volume of the sorber/regenerator units; 2) the sorbent material changes structurally during regeneration; as a result, its sulfur capacity gradually declines; and 3) the regeneration offgas requires treatment to recover sulfur in one form; this is highly undesirable for any power plant; and totally unrealistic for small-scale devices, APUs, and the like.
Remarkably, among others, the present invention solves all of the aforementioned problems: 1) adsorption of H2S under high space velocities allows only the surface of the sorbent to sulfide; very fast regeneration in various gas streams is then used to simply desorb the H2S at times comparable to the adsorption times. Thus, just one sorber/regenerator pair suffices for small- or large-scale fuel cell power plants; 2) only the surface of the sorbent is regenerated/sulfided in cyclic form. This process is reversible, with no irreversible structural complications. The sorbent capacity remains constant after steady-state operation is established; and 3) the sulfur product recovered in the regeneration offgas is approximately 100% H2S, i.e., it requires no further treatment; it can be simply collected in a trap. This approach works over a wide range of temperatures and for all the sorbent materials tested. The disclosures of the foregoing US patents are expressly incorporated herein by reference in their entirety.